The need to install panel fasteners occurs throughout the aerospace industry as well as in other industries. The process for installing a fastener varies, but often involves some combination of the following steps: clamping the structure to be fastened together; drilling a hole through the structure; coldworking the hole; reaming, chamfering, and countersinking the hole; inspecting the hole; applying sealant and feeding the fastener to the hole; inserting the fastener in the hole; upsetting the fastener if it is a rivet, or feeding a nut or collar to the installed bolt and torqueing the nut or swaging the collar; shaving the head of the fastener head to insure flushness with the panel; and unclamping the fastened structure.
Automatic fastening machines based on a large C-frame design are widely used in the prior art and offer the capability to arrange a few tooling functions, such as drilling, fastener insertion, and fastener shaver, on a linear positioning carriage. This design is inherently limiting. The carriage typically translates a maximum of about two feet into or out from the C-frame depending on which of the three tools is desired for use. As the carriage moves, the moment that the inertia of it's mass applies to the C-frame causes some frame distortion and vibration. The distortion causes the tools located on the C-frame to contact the part at slightly different angles and at slightly different points. The (normality of the tools at their contact with the workpiece can be corrected with normality correction mechanisms, but this can increase the cost and complexity of the machine and of the programming of the machine control system. While it is possible to build a larger carriage, this requires a more robust and expensive C-frame. A better system would provide a lighter, yet equally precise, tool indexing system that would be less likely to cause C-frame distortion.
Not only does the linear positioning carriage constrain function options and accuracy, it also limits process speed. A linear positioning carriage is slow because of the relatively long distances which it must travel when indexing between tooling locations. The large mass of the cariage and the tools mounted thereon can generate inertial forces when the carriage is moved, especially when it is moved rapidly in an effort to decrease cycle time. These inertial forces can cause vibration or oscillation of the C-frame which must be given time to damp out before attempting processing operations with the tools on the workpiece, thereby lengthening cycle time. The portion of process flow time spent changing tools using conventional linear carriage indexing systems often exceeds seventy percent of the total flow time necessary for fastener installation. Thus, even if a linear positioning carriage could be devised with sufficient structural rigidity to accommodate all the tools desired for fastener installation, the amount of process time spent indexing between these tools could require several fastening stations working simultaneously to achieve the desired production rates. An improved system would reduce the portion of process time consumed in indexing between tools.
Prior art linear positioning carriages require considerable effort to align each tool. Each tool must be adjusted along one linear axis and two rotational axes. Because the carriage travels in only one direction, tools must be shimmed normal to the direction of travel so that the work tip of each tool passes over the same point. The tool must also be rotationally adjusted both in the plane of travel and normal to the plane of travel so that each tool acts along the same axis. Play in the tool-to-carriage mounts allows adjustment in the plane of travel, and shims are used to adjust rotation normal to the plane of travel. A preferred system would simplify tool setup.
The ideal solution would be a fastening system that allows use of several kinds of fasteners such as rivets, bolts with torqued on nuts, and lockbolts with swaged on collars, and one that allows all operations from drilling the fastener hole to shaving the head of the installed fastener without repositioning the fastening station relative to the panel. Because a large number of expensive C-frame systems are currently in use, such a system should be sufficiently dimensionally compact to allow retrofitting the improved system into existing machines.
Tooling turrets have been used in the prior art to position tooling for sequential operations on keyboard assembly systems, as shown in Suzuki et al., U.S. Pat. No. 4,656,726; drilling systems, as shown in Azizi et al., U.S. Pat. No. 4,587,703 and other applications. However, insuperable obstacles have been seen as making impractical or impossible the use of a turret in a C-frame riveter for installing aircraft quality fasteners. Several sequential operations of high precision, such as drilling, coldworking, reaming, and countersinking, are necessary for high quality fastener installation. If the axis of the tools performing these operations do not accurately align with the desired fastener centerline, expensive time consuming rework or part scrap will result. A turret may not precisely locate sequentially positioned tools in the identical position over the fastener hole axis. Moreover, the application of large forces, on the order of 5000 pounds or more, is required for riveting, and the application of such large forces in a rotatable turret system can complicate the turret design. It would be desirable if the tool position could be accurately and repeatably established such that the tooling center for each tool on the turret could be made to coincide with the same point on the part. Thus, process speed would be increased both by having all operations occurring without repositioning or changing tooling, and by accurately installing fasteners, eliminating poorly installed fasteners.
Thus, it would be desirable to create a fastener installation system that could transfer all tooling required for aircraft skin panel hole preparation and fastener installation without shifting the fastening station relative to the panel between operations. Further, because the industry has a large number of commonly designed C-frame fastener systems, the ideal design would replace existing tool carriages with improved carriages without replacing the entire system. It would be relatively compact and require little mass translation during sequential positioning of tools to avoid deflection and oscillation of the C-frame, thereby making possible a short cycle time. It would have the accuracy required for aircraft quality fasteners, enable the application of large magnitude forces for riveting, and require little set-up time by providing a positioning system for accurately indexing the turret based tooling relative to fastener locations regardless of their mounting position on the turret.